1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
|
#include "pathlocks.hh"
#include "util.hh"
#include <cerrno>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#ifdef __CYGWIN__
#include <windows.h>
#include <sys/cygwin.h>
#endif
namespace nix {
int openLockFile(const Path & path, bool create)
{
AutoCloseFD fd;
#ifdef __CYGWIN__
/* On Cygwin we have to open the lock file without "DELETE"
sharing mode; otherwise Windows will allow open lock files to
be deleted (which is almost but not quite what Unix does). */
char win32Path[MAX_PATH + 1];
cygwin_conv_to_full_win32_path(path.c_str(), win32Path);
SECURITY_ATTRIBUTES sa; /* required, otherwise inexplicably bad shit happens */
sa.nLength = sizeof sa;
sa.lpSecurityDescriptor = 0;
sa.bInheritHandle = TRUE;
HANDLE h = CreateFile(win32Path, GENERIC_READ | GENERIC_WRITE,
FILE_SHARE_READ | FILE_SHARE_WRITE, &sa,
(create ? OPEN_ALWAYS : OPEN_EXISTING),
FILE_ATTRIBUTE_NORMAL, 0);
if (h == INVALID_HANDLE_VALUE) {
if (create || GetLastError() != ERROR_FILE_NOT_FOUND)
throw Error(format("opening lock file `%1%'") % path);
fd = -1;
}
else
fd = cygwin_attach_handle_to_fd((char *) path.c_str(), -1, h, 1, O_RDWR);
#else
fd = open(path.c_str(), O_RDWR | (create ? O_CREAT : 0), 0666);
if (fd == -1 && (create || errno != ENOENT))
throw SysError(format("opening lock file `%1%'") % path);
#endif
return fd.borrow();
}
void deleteLockFilePreClose(const Path & path, int fd)
{
#ifndef __CYGWIN__
/* Get rid of the lock file. Have to be careful not to introduce
races. */
/* On Unix, write a (meaningless) token to the file to indicate to
other processes waiting on this lock that the lock is stale
(deleted). */
unlink(path.c_str());
writeFull(fd, (const unsigned char *) "d", 1);
/* Note that the result of unlink() is ignored; removing the lock
file is an optimisation, not a necessity. */
#endif
}
void deleteLockFilePostClose(const Path & path)
{
#ifdef __CYGWIN__
/* On Windows, just try to delete the lock file. This will fail
if anybody still has the file open. We cannot use unlink()
here, because Cygwin emulates Unix semantics of allowing an
open file to be deleted (but fakes it - the file isn't actually
deleted until later, so a file with the same name cannot be
created in the meantime). */
char win32Path[MAX_PATH + 1];
cygwin_conv_to_full_win32_path(path.c_str(), win32Path);
if (DeleteFile(win32Path))
debug(format("delete of `%1%' succeeded") % path.c_str());
else
/* Not an error: probably means that the lock is still opened
by someone else. */
debug(format("delete of `%1%' failed: %2%") % path.c_str() % GetLastError());
#endif
}
bool lockFile(int fd, LockType lockType, bool wait)
{
struct flock lock;
if (lockType == ltRead) lock.l_type = F_RDLCK;
else if (lockType == ltWrite) lock.l_type = F_WRLCK;
else if (lockType == ltNone) lock.l_type = F_UNLCK;
else abort();
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0; /* entire file */
if (wait) {
while (fcntl(fd, F_SETLKW, &lock) != 0) {
checkInterrupt();
if (errno != EINTR)
throw SysError(format("acquiring/releasing lock"));
}
} else {
while (fcntl(fd, F_SETLK, &lock) != 0) {
checkInterrupt();
if (errno == EACCES || errno == EAGAIN) return false;
if (errno != EINTR)
throw SysError(format("acquiring/releasing lock"));
}
}
return true;
}
/* This enables us to check whether are not already holding a lock on
a file ourselves. POSIX locks (fcntl) suck in this respect: if we
close a descriptor, the previous lock will be closed as well. And
there is no way to query whether we already have a lock (F_GETLK
only works on locks held by other processes). */
static StringSet lockedPaths; /* !!! not thread-safe */
PathLocks::PathLocks()
: deletePaths(false)
{
}
PathLocks::PathLocks(const PathSet & paths, const string & waitMsg)
: deletePaths(false)
{
lockPaths(paths, waitMsg);
}
void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
{
/* May be called only once! */
assert(fds.empty());
/* Note that `fds' is built incrementally so that the destructor
will only release those locks that we have already acquired. */
/* Sort the paths. This assures that locks are always acquired in
the same order, thus preventing deadlocks. */
Paths paths(_paths.begin(), _paths.end());
paths.sort();
/* Acquire the lock for each path. */
for (Paths::iterator i = paths.begin(); i != paths.end(); i++) {
checkInterrupt();
Path path = *i;
Path lockPath = path + ".lock";
debug(format("locking path `%1%'") % path);
if (lockedPaths.find(lockPath) != lockedPaths.end())
throw Error("deadlock: trying to re-acquire self-held lock");
AutoCloseFD fd;
while (1) {
/* Open/create the lock file. */
fd = openLockFile(lockPath, true);
/* Acquire an exclusive lock. */
if (!lockFile(fd, ltWrite, false)) {
if (waitMsg != "") printMsg(lvlError, waitMsg);
lockFile(fd, ltWrite, true);
}
debug(format("lock acquired on `%1%'") % lockPath);
/* Check that the lock file hasn't become stale (i.e.,
hasn't been unlinked). */
struct stat st;
if (fstat(fd, &st) == -1)
throw SysError(format("statting lock file `%1%'") % lockPath);
if (st.st_size != 0)
/* This lock file has been unlinked, so we're holding
a lock on a deleted file. This means that other
processes may create and acquire a lock on
`lockPath', and proceed. So we must retry. */
debug(format("open lock file `%1%' has become stale") % lockPath);
else
break;
}
/* Use borrow so that the descriptor isn't closed. */
fds.push_back(FDPair(fd.borrow(), lockPath));
lockedPaths.insert(lockPath);
}
}
PathLocks::~PathLocks()
{
for (list<FDPair>::iterator i = fds.begin(); i != fds.end(); i++) {
if (deletePaths) deleteLockFilePreClose(i->second, i->first);
lockedPaths.erase(i->second);
if (close(i->first) == -1)
printMsg(lvlError,
format("error (ignored): cannot close lock file on `%1%'") % i->second);
if (deletePaths) deleteLockFilePostClose(i->second);
debug(format("lock released on `%1%'") % i->second);
}
}
void PathLocks::setDeletion(bool deletePaths)
{
this->deletePaths = deletePaths;
}
}
|
